Two-way radio systems have become extremely popular in recent years, and so much so that usual prior art forms of voice signalling are no longer adequate. Digital and tone signalling equipment are presently in widespread use to provide selective calling, especially when the radio system is to be interconnected to a telephone exchange. The "Touch-Tone" system developed by American Telephone and Telegraph Company for its telephone switching networks has proven to be ideally adaptable to radio communication.
The usual telephone Touch-Tone encoder is designed so that when a particular pushbutton on the keyboard is depressed, a pair of signalling tones is produced, with each tone being carefully selected to avoid the principal harmonics normally created in speech and music. Touch-Tone signalling has been adapted to radio communication, as stated above, and has been used satisfactorily in the prior art in two-way radio systems.
However, a problem inherent in the prior art Touch-Tone encoders used in conjunction with radio transmitters is that as long as a particular pushbutton is depressed a continuous dual tone is produced. When such a dual tone is transmitted to the receiving equipment over a poor radio transmission path, it can exhibit intermittent characteristics. This creates a false selection effect, since it simulates the condition in which a particular pushbutton has been depressed several times. This problem is overcome in the system described in the above-mentioned copending application, which includes a timing circuit connected to the encoder which serves to cut off each dual tone a short time after it has been produced, so as to make the duration of each dual tone generated by the encoder relatively short and independent of the length of time during which the button on the keyboard which caused the encoder generator to produce the tone was depressed.
Another problem in the prior art Touch-Tone encoders as applied to radio communication is that, unlike usual telephone equipment, no provision is made in the prior art to enable the user to hear the tones being generated. Therefore, the user of the prior art equipment has no way of knowing whether the dual tones have been generated. This latter problem is also solved in the system of the copending application by providing an amplifier which is connected to the encoder generator, and which raises the level of the dual tones generated by the encoder generator to a level at which they may be reproduced by a speaker for monitoring purposes.
A further feature of the encoder of the copending application is the provision of readily accessible controls for setting the monitoring level and the transmission level of the dual tones generated by the encoder, these controls being accessible through respective holes in the encoder housing, so that the settings may be achieved without the necessity for dismantling the unit.
The present invention provides an improved unit which includes a keyboard and associated printed circuit board, and a housing therefor, and which is adapted to incorporate a further printed circuit board containing the encoder circuitry and further circuitry embodying one or more of the features of the system described in the copending application.
The unit of the invention facilitates the manufacturing process, because different printed circuit boards may be selectively incorporated into the unit depending upon which, if any, of the features of the system of the copending application are desired in any particular unit.